Material Sensitivity Study on Lumbar Motion Segment (L2-L3) Under Sagittal Plane Loadings Using Probabilistic Method

OBJECTIVE

In this study, the probabilistic responses of a three-dimensional finite element L2-L3 motion segment, with and without posterior elements, tested under sagittal plane loadings, are presented. Understanding the effect of biologic uncertainties and variations on the biomechanical response provides an insight into spinal behavior under normal and degenerated conditions.

METHODS

The biologic variability of 19 spinal components (nucleus, annulus, ligament, cortical/cancellous bone, endplate, and ligaments) in the motion segment was incorporated using statistical distributions into the model. A total of 2000 runs were performed using Monte Carlo probabilistic algorithms to compute the probabilistic response.

RESULTS

This study establishes the relative importance of the spinal components in resisting the loading modes. The results show that for an intact motion segment, posterior ligaments are more dominant than intervertebral disc in resisting flexion moment. In extension, the capsular ligaments were found to be the most influential parameter. The intervertebral disc (ie, nucleus and annulus) affects the angular response of the disc body segment more than the hard tissues (ie, cortical and cancellous bone).

CONCLUSIONS

The application of the probabilistic analysis provides a new approach whereby the influences of inherent uncertainties and variations in biologic structures can be studied and the biomechanical response assessed.

The influence of tibial and femoral rotation on patellofemoral contact area and pressure

Fixed rotation of either the femur or tibia has a significant influence on the patellofemoral joint contact areas and pressures. This is due to the anatomic asymmetry in the knee with respect to all planes, as well as the laterally directed force vector that naturally exists in bipedal lower-limb biomechanics. Specifically, femoral rotation results in an increase in patellofemoral contact pressures on the contralateral facets of the patella, and tibial rotation results in an increase in patellofemoral contact pressures on the ipsilateral facets of the patella. This difference can be elucidated when one considers that rotation of the femur is biomechanically different than rotation of the tibia. For both tibial and femoral rotations, the patella's distal attachment to the tibial tubercle influences the direction of patellar movement. The biomechanical evidence reviewed in this manuscript suggests that the determining factor in patellofemoral pathology is the derangement of normal joint mechanics. However, despite considerable experimental data supporting this position, there also are theories that suggest otherwise. This illustrates a very important point in patellofemoral joint pathology, where no one factor may be the sole defining etiology. Instead, the patellofemoral joint is one of the most complex diarthrodial joints in the body and there are a number of etiologic factors that can lead to pathology. This should be considered for developing repair and rehabilitation strategies.